This invention relates to a synthetic quartz glass substrate for use as a photomask and a method for preparing the same.
Conventional aligners for photolithography are generally classified into a transmission optical system and a reflection optical system.
When unpolarized plane waves are transmitted by a synthetic quartz glass substrate, the waves divide into two plane-polarized waves which are orthogonal to each other, owing to the birefringence caused by thermal or dynamic stresses in the glass. Since these two plane-polarized waves propagate through the glass at different velocities, they are different in phase upon emergence from the glass, becoming elliptical polarization. When the light transmitted by the substrate is reflected by a mirror of the reflection optical system, a difference in reflectance arises at the mirror surface because the two plane-polarized waves have different electromagnetic vectors. Then the optical intensity of reflected light varies if the birefringence is variant within the substrate.
This phenomenon is further enhanced where a concave mirror is used for reflection because the light reflectance which is affected by the incident angle of light varies among different positions on the concave mirror.
Therefore, if a synthetic quartz glass substrate having a noticeable birefringence is used as the photomask in the aligner based on the reflection optical system, there arises the phenomenon that the light intensity varies over the wafer. Therefore, when the aligner based on the reflection optical system is used, the light sensitivity of a resist coated on a silicon wafer varies within its plane, preventing uniform patterning. The problem becomes more serious as the line width of the pattern becomes narrower.
A variation of light intensity due to birefringence also occurs in the aligner based on the transmission optical system, though the influence is small as compared with the above reflection optical system. Its influence on the exposure of a miniature pattern is of some concern.
It is known that the birefringence in glass is caused by residual stresses therein (see xe2x80x9cThe World of Quartz Glass,xe2x80x9d Kogyo Chosakai). To solve such problems, it is generally practiced to effect annealing treatment on a synthetic quartz glass block, by holding the block at a temperature above the annealing point for a time and gradually lowering the temperature below the strain point for reducing thermal residual stresses (see Glass Handbook, Asakura Publishing K.K.). The annealing treatment is followed by slicing, chamfering, and polishing, thereby producing a synthetic quartz glass substrate.
However, even if such a process is employed, the completed synthetic quartz glass substrate for photomask use has a birefringence which is not only as high as about 20 nm/cm where it is the highest, but also largely varies within the photomask substrate. If this synthetic quartz glass substrate is used as a photomask, a variation of light intensity occurs on the wafer surface, especially in the aligner of the reflection optical system, resulting in a failure to define a miniature circuit with a constant line width.
Therefore, there is a desire to have a synthetic quartz glass substrate having a minimized birefringence and useful as a photomask.
An object of the invention is to provide a synthetic quartz glass substrate for use as a photomask which has a minimized birefringence and which when used as a photomask on a wafer, allows exposure light to distribute at a uniform intensity over the wafer for ensuring a uniform light intensity within the wafer plane for achieving a high patterning precision. Another object of the invention is to provide a method for preparing the synthetic quartz glass substrate.
It has been found that when a synthetic quartz glass member having a higher hydroxyl content in a peripheral portion than in a central portion is annealed, the resulting synthetic quartz glass is suitable as a substrate for photomask use. The annealing step results in the member in which the properties of the peripheral portion are locally altered. The substrate is prepared by machining off the peripheral portion of the annealed member, and slicing the member into a plate shaped substrate, followed by chamfering and etching. The synthetic quartz glass substrate thus obtained has a minimized birefringence, typically up to 2 nm/cm. When used as a photomask on a wafer in an aligner of the reflection optical system, it allows exposure light to distribute at a uniform intensity over the wafer. This ensures a uniform light intensity within the wafer plane, achieving a high precision of patterning.
According to a first aspect, the invention provides a synthetic quartz glass substrate for use as a photomask, obtained by annealing a synthetic quartz glass member having a higher hydroxyl content in a peripheral portion than in a central portion. Preferably, the synthetic quartz glass has a birefringence of up to 2 nm/cm.
According to a second aspect, the invention provides a method for preparing the above-defined substrate, comprising the steps of annealing a synthetic quartz glass member having a higher hydroxyl content in a peripheral portion than in a central portion, and cutting the member into a plate shaped substrate. The method preferably includes the steps of machining the annealed member to remove the peripheral portion whose properties have been locally altered by the annealing step, cutting the member into a plate shaped substrate, chamfering the substrate, and etching the substrate.